This invention relates in general to containers, and more particularly to containers which are closed by membrane-type seals and a process for applying such seals.
Many food containers are closed by a lid which may be removed and replaced with relative ease, and to insure that the container is completely sealed beneath the lid, it is not uncommon to interpose a disk-like seal element between the lid and the top rim of the container. This seal element conforms to irregularities in the upper rim of the container so that the container is sealed even though its lid does not fit tightly against the upper surface of the rim. Seal elements of this nature are used quite often in connection with screw caps, and are retained in place usually by nothing more than the closure force exerted by the screw cap. Sometimes the disk-like seal elements contain a layer of aluminum foil.
Plastic containers capped with snap-type plastic lids are now used quite extensively in the food industry, particularly for spreads such as margerines. Both the container and lid are quite flexible, but when the lid is in place, the upper end of the container is confined and not easily distorted. The snap lids cannot be run down and secured with the firmness of a screw cap, and it is therefore difficult to adequately compress a disk-like seal with such a lid. Moreover, heat sealing foil membranes to the top rims of such containers has not been practical since the containers will distort significantly under the force of a heated platen. Of course, the lid will hold the container in the proper shape, but the lid cannot be in place when a weld is made with a heated platen.
U.S. patent application Ser. No. 672,719, of David S. Knudsen filed Apr. 1, 1976, and entitled MACHINE AND PROCESS FOR CAPPING AND SEALING CONTAINERS, discloses a machine for heat sealing foil membranes to the top rims of highly flexible containers while the lids of the containers are in place, thus enabling the lids to rigidify and maintain the containers in the proper place during the application of the force necessary to effect the heat seal. However, the foil membrane is not heated by conduction but is instead inductively heated in a strong magnetic field, the polarity of which constantly changes.